1. Field of the Invention
The present invention relates to a heat transfer tube for an absorber having an improved absorption performance including a plurality of crest portions and trough portions on the outer surface thereof for use with an absorber in an absorption heat exchanger such as an absorption refrigerating machine, an absorption cooling and heating machine, etc..
2. Description of the Related Art
In an absorption heat exchanger such as an absorption refrigerating machine, the heat exchanger is interiorly held in vacuum, a refrigerant is evaporated at a low temperature, and a cold water is removed by the evaporation latent heat to use the cold water for air conditioning or the like.
An absorber and an evaporator are integrally housed in the body. To continuously obtain evaporation, a refrigerant vapor generated in the evaporator is absorbed in an absorbing liquid scattered on the surface of the heat transfer tube of the absorber to maintain the interior of the body at a given vacuum degree. Accordingly, it is necessary for promoting the refrigeration efficiency of the absorption refrigerating machine and the absorption cooling and heating machine to increase the quantity of generation of the refrigerant vapor in the evaporator and the quantity of absorption, that is, the absorption efficiency. For increasing the absorption efficiency, the promotion of performance of the heat transfer tube is most effective means. Heat transfer tubes having various shapes have been studied and proposed.
For example, in the techniques disclosed in Japanese Utility Model Application Laid-Open No. Hei 2-89270 and Japanese Patent Application Laid-Open No. Hei 2-176378, longitudinal grooves which are continuous in an axial direction of the tube, and crest portions and trough portions formed in a direction at right angles to the axis of the tube have a shape comprising a curvature in a predetermined relationship.
These arts have a feature of not impairing waving of an axial absorbing liquid generated by Marangoni convection. Further, when the absorbing liquid passes from the crest portions to the trough portions, the further turbulence effect is obtained.
Further, the technique formed with intermittent crest and trough portions is disclosed in Japanese Utility Model Publication No. 46-67080 and Japanese Patent Publication No. Hei 5-22838. These arts have a feature that the absorbing liquid is stirred by the intermittent crest and trough portions and the time of residence is prolonged.
However, while the aforementioned conventional arts, the heat transfer performance is promoted to some extent, there were various problems as mentioned below.
First, in the heat transfer tube having the shape in which there is provided the groove which is continuous in the axial direction of the tube, a difference in heat transfer performance occurs depending on the direction of installation of the tube.
More specifically, in the case where the heat transfer tube is arranged so that the trough portions are positioned vertically and upwardly, the absorbing liquid tends to stay in the trough portions so that the absorbing liquid is not well discharged. Therefore, the absorbing liquid whose absorbing efficiency is lowered stays in the trough portions, resulting in the lowering of the heat transfer performance. Further, when the flow rate of the absorbing liquid increases, the absorbing liquid 5 sometimes drops out in the crest portion at the lower part of the tube, in which case also, the heat transfer performance lowers. To prevent these injurious effects, it is effective to arrange a row of groups of tubes so that the crest portions are positioned up. In this case, however, in operation of inserting tubes into the refrigerating machine, it is necessary to proceed the operation while making sure directions one by one, thus imposing a great burden on an operator.
Further, when the wall thickness of the heat transfer tube is decreased, the heat transfer tube is twisted when the former is secured to a tube plate, worsening the distribution of the absorbing liquid to sometimes lower the performance.
Furthermore, since when the depth of the trough portion becomes deepened, the quantity of residence of the absorbing liquid increases, the necessary circulating quantity of the absorbing liquid for driving a refrigerating cycle increases to increase the weight of the machine.
Next, the heat transfer tube disclosed in Japanese Utility Model Publication No. 46-67080 has the intermittent trough portions. As shown in FIG. 6, trough portions 8 are arranged intermittently in a peripheral direction of a tube to constitute a row of a group of trough portions, which is different in circumferential position of the trough portions from that of the adjacent row, and the circumferential positions of the trough portions are superposed on every other row to arrange the trough portions 8. However, in this conventional heat transfer tube, there is present a web-like area where no trough portion is present as viewed axially. For this reason, when the Marangoni convection occurs as the refrigerant vapor is absorbed, the flowing down absorbing liquid rises in a stripe form and flows down while waving in an axial direction of the tube. Therefore, the absorbing liquid does not flow into the trough portions as shown in FIG. 6 depending on places. As a result, there is a disadvantage in that the residence of the absorbing liquid is not sufficient and the absorbing performance will not be promoted.
The heat transfer tube disclosed in Japanese Patent Publication No. Hei 5-22838 relates to an improvement in the construction of the heat transfer tube disclosed in the aforementioned Japanese Utility Model Publication No. 46-67080 but has the problems as follows. That is, the heat transfer tube of Japanese Patent Publication No. Hei 5-22838 has the construction contemplated so that the absorbing liquid can be stayed on the surface of the tube for a period of time as long as possible, in which the absorbing liquid does not pass the protrusions provided intermittently but the absorbing liquid flows down while going round a flat portion between the protrusions.
By the construction as described above, it is possible to prolong the time of residence of the absorbing liquid and increase the quantity of residence of the absorbing liquid. However, since the absorbing liquid stays on the surface of the tube more than as needed, the necessary circulating quantity of the absorbing liquid increases to increase the weight of the machine, as previously mentioned. Further, since a flowpassage of the absorbing liquid is determined by the trough portions and the absorbing liquid does not flow down passing over the top of the protrusions, the top of the protrusion does not contact with the absorbing liquid. Accordingly, a heat transfer area of the heat transfer tube cannot be effectively secured, and there is a limit to promote the heat transfer performance.
In view of the foregoing, the present inventors have proposed a heat transfer tube for an absorber which further promotes the heat transfer performance and promotes the workability when tubes are assembled into a refrigerating machine (Japanese Patent Application Laid-Open No. Hei 8-159605).
According to the above-mentioned prior application, there is provided a heat transfer tube for an absorber for use with an absorber having a plurality of tubes arranged horizontally, characterized in that in a row of trough portions adjacent to each other in a circumferential direction of the tube, a center of one row of trough portions coincides with a center between the other row of trough portions in an axial direction of the tube, the ratio L0/L1 of a length L0 of a superposing portion of the trough portions in the rows adjacent to each other in a circumferential direction of the tube to a length L1 of the trough portions is set in the range of 0.2 to 0.8, the ratio W1/W2 of a width W1 in a circumferential direction of the trough portions to a width W2 in a circumferential direction of the tube of crest portion between the trough portions is set in the range of 0.5 to 2.5, a depth h of the trough portion is set in the range of 0.5 to 1.5 mm, and a length L of the trough portion is set in the range of 10 to 50 mm.
In the thus configured heat transfer tube for an absorber, the row of the intermittent trough portions extending in an axial direction of the tube is arranged so that the ratio of a length of one row of trough portions to a superposing length with the other row of trough portions adjacent thereto has a predetermined value. In the longitudinal groove tube having grooves continuous in an axial direction of the tube, there occurs an unevenness in performance depending on the direction of installation, as previously mentioned. However, in the heat transfer tube for an absorber having the intermittent trough portions, there is no directivity, and even if the upper surface of the tube is arranged in a suitable direction, a substantially given heat transfer performance is exhibited.
Further, a determined flowpassage for the absorbing liquid is not formed but the absorbing liquid flows down while uniformly wetting the tube wall, thus obtaining a high absorption performance.
However, the heat transfer tube for an absorber according to the aforementioned prior application can achieve the intended object, but the quantity of residence of the absorbing liquid on the surface of the tube is less, and the absorption performance is not always sufficient. Because of this, developments of a heat transfer tube for an absorber having a further excellent absorption performance have been desired.